In four wheel drive vehicles, it is desirable to provide a drive torque to each wheel in relation to the vehicle load on each wheel and the road surface friction coefficient for each wheel to maximize vehicle traction efficiency and reduce chance of tire spin. When there are equal road surface friction coefficients for all four wheels, which is usually the case, minimum wheel spin occurs when the drive torque to each wheel is proportional to the vehicle loading on each wheel.
The state of the art technology for achieving ideal wheel torque proportioning is to use a locked differential between the front and rear axles which results in identical front and rear wheel speeds. Providing identical front and rear wheel speeds is a very effective way of ensuring ideal proportioning of wheel torque and, consequently, tractive force.
For vehicles in which there is no mechanical linkage between the front and rear wheels or any of the wheels, e.g., certain dual engine and electric vehicles, the use of a locked differential for ideal torque proportioning is not applicable. What is desired is a control system for a four wheel vehicle with an independent drive train for each wheel which will provide ideal wheel torque proportioning and ideal tractive force proportioning.